In the processing of wood chips preparatory to introduction to a digester, it is preferred to reprocess chips which are thicker than a predetermined thickness (commonly about 8 mm) and to discard those chip particles which have fibers shorter than a preset minimum length or which are in the form of flakes thinner than a preset thickness, because these are relatively poor digesting materials. For purposes of the present description, the chips to be reprocessed will be called "over-thick" and the chip particles with overly short fibers and flakes will be called "fines."
"Pins" are an additional category of particles in wood chip material which it is preferred to take into consideration in many pulping operations since they do not result in as high a quality of pulp as do normally acceptable chips and can cause plugging in the digester. Pins may be classified as a toothpick-like material which will pass through a 7 mm round hole on a Statsvets-type separator, but not through a 3 mm round hole. When pins are classified in this manner, fines are considered to be sawdust-like material that, when wet, will pass through a 3 mm round hole on a Statsvets-type classifier.
The advantage of initially separating a relatively large percentage of the pins is that they may be accurately blended in a predetermined amount with the otherwise acceptable chips if they are to be used in the pulping operation, or can be discarded in whole or in part with the fines. In operations in which the pins are not separated as a class, the smaller pins become part of the fines and the remaining pins become part of the acceptable chips.
The screening of wood chip material is not only difficult because of the presence of fines and pins, but it is also complicated by the fact that the chips normally vary in length from about 20 to 30 mm and in width from about 15 to 20 mm. Thus, the thickness of the chips is usually considerably smaller than the other dimensions.
The traditional screening apparatus for pulp chips have been (a) sloped, vibratory holed screens given an oscillating or circular motion, commonly in the range of 2 to 3 inches, at a relatively high speed to shift the properly sized chips through the holes in the screen, and (b) disk screens such as that shown, for example, in U.S. Pat. No. 4,301,930, which comprises a bed of parallel, co-rotating shafts carrying interdigitated disks having a clearance defined by the maximum chip thickness to be tolerated.
Disk screens have been considered by many in the cellulose industry as superior to vibratory screens; but as indicated in U.S. Pat. No. 4,660,726, disk screens have a relatively low screening capacity per square meter of screening surface, and, as indicated in U.S. Pat. No. 4,538,734, it is very difficult to attain and maintain uniform slot widths between the disks of a disk screen, particularly when the slot widths are required to be so narrow. As a consequence, there have been efforts to provide improved techniques for mounting and replacing the disks of disk screens and attempts to develop a suitable alternative to disk screens. Such attempts have included oscillating bar screens, such as shown in U.S. Pat. No. 4,660,726, and synchronously driven, intermeshing screw spirals, such as disclosed in U.S. Pat. No. 4,430,210.